1. Field of the Invention
The present invention relates to a method for coastal protection where the coastal area has an underlying freshwater basin and below this a salt water tongue which extends obliquely down into the coastal profile.
2. The Prior Art
For coastal protection, it is generally known to build breakwaters of huge stones or concrete blocks which extend from the beach to a distance into the water. Breakwaters are effective, but the costs of construction and maintenance are relatively great. Another coastal protection method is coastal feeding where large amounts of sand are transported to the stretch of coast which is to be protected. This method also involves great costs of construction and maintenance, since large amounts of sand have to be transported. These two methods are still the most widely used coastal protection methods.
In connection with the establishment of intakes for the pumping of sea water for use in salt water aquarias, it was discovered in the early 1980s that sedimentation took place around the intake, which became clogged because of the deposits on top of the intake. This was the incentive for experimenting with a new method for coastal protection, as described in DK 152 301 B. The idea of the method is to pump water from drains established along the shore line, resulting in sedimentation at the drains. However, this method never found extensive use, as it requires a great pumping capacity and consequently high costs of construction and high pump operating costs.
U.S. Pat. No. 5,294,213 discloses a similar system likewise based on drainage pipes established in parallel with the coastal both on the beach and in the water. The operation of the system, which is likewise based on pumping of water, is adapted to the weather, i.e., whether ordinary water level, low water, high water or storm conditions. The system includes a water reservoir into which the water may be pumped through the drainage pipes, and water may be pumped through these into the sea, e.g., to remove sand banks formed by a storm.
A corresponding method is known from U.S. Pat. No. 4,898,495 to keep an inlet, which debouches into the sea, open. This method is likewise based on pumps. The system comprises various diffuser arrangements to remove deposits from the mouth of the inlet by fluidizing these and transporting the material further downstream of the inlet mouth by generating a flow. Sedimentation is carried out downstream of the inlet mouth by pumping water from drains to the diffuser arrangements.
An object of the present invention is to provide a method for coastal protection which is not vitiated by the drawbacks of the known coastal protections.
This is achieved according to the invention by a method which is characterized in that the pressure of the groundwater basin at least along an area at the shore line is equalized completely or partly through pressure equalization modules, preferably in the form of pipes with a filter at the bottom, which extend down into the groundwater basin.
It has surprisingly been found by the invention that positioning of pressure equalization modules in the beach results in sedimentation of material at the area where the modules are placed.
A possible explanation as to why coastal accretion takes place is that the very fine sand which is fed to the profile partly by the sea and partly by the wind and which is packed with silt and other clay particles, reduces the hydraulic conductivity. Deeper layers in the coastal profile, which have exclusively been built by the waves of the sea, are primarily coarse in the form of gravel and pebbles which have a greater hydraulic conductivity. The difference in hydraulic conductivity will be seen clearly when digging into a coastal profile, it being possible to dig a hole in the profile, and the groundwater will then rise up into the profile once the water table is reached. The reason is the very different hydraulic conductivity and that the freshwater is under pressure from the hinterland. Thus, the coastal profile may be compared to a downwardly open tank where the tank is opened at the top with the pressure equalization modules which extend through the compact layers of the profile so that the water runs more easily and thereby more quickly out of the profile in the period from flood to ebb. This means that a pressure equalized profile is better emptied of freshwater and salt water in the fall period of the tide. When the tide then rises from ebb to flood, a greater fluctuation occurs in the foreshore, as the salt water in the swash zone is drained in the swash zone so that materials settle in the foreshore during this period of time. Conversely, coastal erosion takes place if the freshwater is under pressure in the foreshore, as the salt water will then run back into the sea on top of the freshwater and thereby erode the foreshore. In reality, the pressure equalization modules start a process which spreads from the pressure equalization modules, as the silt and clay particles are flushed out of the foreshore when the fluctuation is increased because of the draining action of the modules. Further, a clear connection has been found between the amount of sediment transport on the coast and the rate of the coastal accretion. It has been found that the pressure equalization modules create a natural equilibrium profile with a system of about 1:20, so that the waves run up on the beach and leave material, as water in motion can carry large amounts of material which settle when the velocity of the water decreases. The profile must therefore have a given width with respect to the tide and a maximum water level in the area. Coastal profiles with pressure equalization modules naturally become very wide, which results in a very great sand drift on the foreshore. This great sand drift is utilized by establishing longitudinal fascines high up in the beach and transverse fascines with an increasing height toward the foot of the dune, the fascines forming the upper part of the beach profile.
The invention will be described more fully below with reference to the accompanying drawings.